Grip Force Repeatability in Lathes

You could be losing up to 40 percent of your chuck clamp force in an eight-hour work day. This affects your workpiece and process quality and the safety of your equipment and personnel. Not considering the repeatability of the clamp force in your machining process is a common oversight.

When purchasing a chuck, few people ask the question, “How repeatable is the clamp force of the workholding?” More commonly we ask, “What is the repeatability of the jaw movement?”, because that factor traditionally defines chuck accuracy and its ability to clamp and re-clamp a workpiece while achieving the same centerline or T.I.R. This jaw movement repeatability often defines the manufacturing quality of a chuck.

However, in most cases, since we are only clamping a part one time, the repeatability of the chuck is of less concern than how much force is applied to clamp the workpiece and the degree of deformation being induced into the part.

From a process repeatability perspective (CPK), we should ask, “What is the grip force applied on the first part vs. the last part we clamp in the batch?” If we clamp the first part with 10,000 ft-lb and the last part of with 6,000 ft-lb, our process capability is less stable or repeatable than if we clamp the first and the last part with the same amount of force. By not clamping the parts with the same force we cannot hold position, nor can we predict the deformation of the part.

Often, we clamp our parts with excessive force to compensate for this effect, thus causing more deformation then necessary. This deformation results in out-of-round parts and additional wear on the chuck, causing a loss of accuracy in the centering function. Workpieces should only be clamped with enough force to resist the cutting tool forces and rotational forces. So how does one improve the repeatability of the clamp force in their workholding?

Loss of lubrication is the primary cause of grip force reduction over time. The mechanism that moves the jaws and applies the grip force is normally open to the environment and is losing lubrication with each actuation and through rotation. Many contact and wear surfaces in a chuck need lubrication in order to transfer the force from the actuating cylinder to the clamping jaws. Through actuation, these surfaces lose their lubrication and more force is lost to internal friction and not transferred to the workpiece.

Standard chucks lose lubrication due to rotational forces and coolant wash out. As the chuck rotates, the grease is expelled to the outer surfaces and eventually out of the chuck completely through the small gaps between the chuck body and the master jaws. The highest amount of friction is created in the jaw actuation mechanism which is at the center of the chuck body. The greatest need for lubrication is at the center of the chuck, but all the grease is forced to the outer areas through centrifugal force.

Machines equipped with high pressure coolant increase this problem as the coolant literally pressure-washes the grease from the chuck and forces contaminates into the chuck mechanism. As lubrication intervals increase, more actuating force is lost to friction than is applied to the part through clamping force. The first workpiece is clamped with excessive force and the last workpiece before lubrication may be clamped with too little force, all due to loss of lubrication.

This has a serious impact on repeatability. You could lubricate your chuck with a grease gun for every part, but the time and expense required and the additional contamination of your coolant system with grease would be unacceptable. The advent of the fully sealed chuck solves this problem. Fully sealed chuck technology has been proven around the world in applications as demanding as dry cutting and vertical turning of thin cross-section cast iron brake discs.

One shop had to replace chucks every three months due to the infiltration of cast iron particles and loss of lubrication wearing the chucks out. When they added machines with high pressure coolant, the problem became worse. But with fully sealed chucks they now operate hundreds of thousands of cycles and go over a year without lubricating their chucks or replacing them. The reduction in downtime has been a significant cost savings. Part quality is better because of the repeatability of the grip force over an extended period.

A fully sealed chuck is created through the integration of specially designed chuck bodies, seals and covers into the chuck design. Simply adding wipers and O-rings to a chuck is not enough to call it sealed. The chuck must be designed as a sealed system. Keeping contaminants out and grease in is only part of the solution; the chuck must be able to re-lubricate itself each time it is actuated.

Using fully sealed technology, the internal mechanism of the chuck is designed to take grease from the outer surfaces of the body and redistribute it back to the center of the chuck every time the chuck is actuated. The chuck acts as its own grease gun, keeping the operator from having to attend to this task.

Additionally, there is no contamination of the coolant system by grease, reducing maintenance costs on that system as well. The next time you look at your process capability, have a deformation problem, or are concerned about safety or damage from a released part, look at your clamp force repeatability for a possible solution, as well as reducing your downtime and maintenance costs.